Malignant transformation and tumor progression are frequently accompanied by the loss of genes with antitumor activity. One such tumor suppressor gene, PTEN, is located on chromosome 10 at q22-23, a locus that is altered in almost half of all endometrial cancers, in a third of glioblastomas, and in a wide range of other human neoplasms, such as prostate, brain, breast, kidney cancers, and leukemias. In addition, PTEN is mutated in three human inherited cancer-predisposing syndromes. Thus, PTEN appears to play an important role in a pathway the loss of which sensitizes cells to malignant transformation. It has recently become clear that the PTEN protein is a specific phosphatidylinositol 3-phosphatase (PI3Pase) that directly counteracts the versatile effects of phosphatidylinositol 3-kinase (PI3K) in cell growth, survival, motility, cytoskeletal architecture and differentiation. The regulation of PTEN is currently poorly known. This grant will address a novel hypothesis for the regulation of the biological function of PTEN. Specific aim 1 (Transcriptional regulation of PTEN) represents the continuation of our recent finding that the transcription of the PTEN gene is regulated by the Egr-1 transcription factor. Egr-1 is also a tumor suppressor and mediates the upregulation of PTEN in response to UV and g-irradiation and other proapoptotic stimuli. Our work will address our hypothesis that PTEN transcription is influenced by D3-phosphoinositides (the substrates for PTEN) as part of a feedback loop that may include Egr-1 and/or other Egr family members, particularly in lymphoid cells and tumors. The scenarios present in tumors will be addressed. Specific aim 2 (Post-translational regulation of PTEN) will explore a similar model for the regulation of PTEN degradation. Our hypothesis predicts that D3-phosphoinositides influence the turnover of PTEN by a mechanism that involves phosphorylation at S380 (plus adjacent sites), which protects the protein from degradation. We will study this mechanism, identify the responsible kinase (could well be PKB), and the route of proteolysis. We will also evaluate the effects of S380 phosphorylation on other aspects of PTEN function. Taken together, our results are expected to improve our understanding of the molecular mechanisms by which PTEN participates in carcinogenesis and may open new avenues for rational drug design. The possible crosstalk between two pathways of programmed cell death (PTEN and Egr-1) may be highly relevant for the understanding and eventual treatment of a large number of human cancers.